The present invention relates to a wastewater treatment process, and more particularly toward an activated sludge treatment process using immersed membrane filters.
There is an ongoing need for reliable treatment of wastewater both for reuse of the water and in order to meet ever more demanding state and federal discharge quality standards. Of continued concern is the need to remove organic nutrients including phosphates and nitrogen which encourage the growth of water plants and algae which in turn result in degradation of the natural environment and assorted health concerns. The effective and reliable removal of pollutants from wastewaters, particularly carbonaceous materials and nutrients such as nitrogen and phosphorous, has become increasingly important in efforts to supplement and reuse existing municipal water resources. Typical wastewater treatment processes usually include multiple treatment areas that can be broken down into: (1) a preliminary treatment area; (2) a primary treatment area; and (3) a secondary treatment area. The preliminary treatment is concerned with removal of grit and damaging debris, such as cans, bath towels and the like from the untreated wastewater. This is usually a two stage process whereby the debris such as rags and cans are removed by screens and the grit and heavy inorganic solids settle out of the untreated water as is passes through a velocity controlled zone. Dissolved organics and organic nutrients are carried within the fluid stream as it passes from the preliminary treatment area.
A typical primary treatment area, which is an optional element of wastewater treatment, entails a physical process wherein a portion of the organics is removed by flotation and sedimentation. Usually 40-70% of the suspended solids are removed in the primary treatment area.
The secondary treatment area is usually a biological treatment process where bacteria are utilized under controlled conditions to remove nutrients or non-settling suspended solids and soluble organics from the wastewater. These materials would result in an unacceptable biological oxygen demand (xe2x80x9cBODxe2x80x9d) if left untreated. A typical secondary treatment method is an activated sludge process in which the wastewater is aerated and agitated with an activated sludge then purged of a variety of microorganisms. Often this aerobic stage is combined with an anaerobic stage, i.e., a stage operated in the absence of induced oxygen, either soluble or derived from nitrites or nitrates (NOx), and an anoxic system, i.e., where oxygen is absent but nitrate is present. Phosphorus removal is accomplished in the anaerobic stage and de-nitrification is accomplished in the anoxic stage.
Daigger, U.S. Pat. No. 5,480,548, the disclosure of which is hereby incorporated by reference, discloses in considerable detail anaerobic-anoxic-aerobic secondary treatment processes. Daigger teaches a serial biological reactor consisting of an anaerobic zone, an anoxic zone and an aerobic zone. Daigger further teaches the desirability of an anoxic recycle (xe2x80x9cARCYxe2x80x9d) of mixed liquor suspended solids (xe2x80x9cMLSSxe2x80x9d) from an anoxic zone to an upstream anaerobic zone. Daigger teaches that effluent from the bioreactor enters a conventional gravity separator from which treated effluent is decanted and return activated sludge is returned to an upstream anoxic zone. In addition to Daigger, Hawkins, U.S. Pat. No. 5,601,719, Marsman, U.S. Pat. No. 5,342,522, Strohmeier, U.S. Pat. No. 5,798,044, Hong, U.S. Pat. No. 5,650,069, Timpany, U.S. Pat. No. 5,354,471, Wittmann, U.S. Pat. No. 4,961,854, Nicol, U.S. Pat. No. 4,787,978 and Yang, U.S. Pat. No. 5,942,108, each disclose multi-zoned bioreactors with some recycling of flow between the various zones to maintain concentrations of useful microorganisms and to improve biological nutrient removal. In each case, however, these patents teach conventional gravity separation by a clarifier or the like, as discussed above with regard to Daigger. One problem with gravity based separation systems is that the solids concentration must be limited in order to effect acceptable levels of gravity clarification. In addition, these systems do not provide a physical barrier to dangerous pathogens such as Giardia and Cryptosporidium. Furthermore, these processes are ineffective for the removal of a wide variety of dissolved organics.
Tanaka, U.S. Pat. No. 6,007,712, teaches a multi-zoned bioreactor wherein the separation of suspended solids occurs at a membrane module. Tanaka provides that non-permeating water which does not pass through the membrane is returned and circulated to a nitrification or aerobic tank. Tanaka also teaches that the membrane of the membrane filter is preferably hydrophilic to make it more difficult for suspended solids to attach and foul the membrane. Tanaka further provides that periodic back washing xe2x80x9cby means of an air and a permeated liquid when the suspended solids component is attachedxe2x80x9d can be performed to purge the solids from the membrane. Tanaka teaches that recycle to the aerobic zone is required because only one seventh ({fraction (1/7)}) of the water present at the membrane can be filtered, thus requiring that the remaining water be returned upstream. Tanaka is silent whether it would be useful or desirable to recycle water if the membrane module were immersed within a hydrafication (or aerobic treatment) tank. In any event, Tanaka does not teach continuous aeration of the membrane module that would provide an oxygen charged MLSS for recycle to an aerobic zone. Tanaka also fails to provide for suitable removal of shock loads of organisms, such as may result from a toxic spill.
Anselme, U.S. Pat. No. 5,364,534, discloses a process for purifying and filtering water including introducing a pulverulent reagent, such as activated carbon, into a water stream downstream of a gravity separation and upstream of a membrane separation. The pulverulent reagent is recycled from the purge of the membrane separation to upstream of the gravity separation. Anselme does not provide for removal of biological nutrients and BOD and is thus of limited utility in treating municipal wastewater and many other sources of biological nutrient and dissolved organics containing waste water.
The present invention is directed toward overcoming one or more of the problems discussed above.
A first aspect of the present invention is an apparatus using activated sludge for the removal of biological nutrients from a wastewater. The apparatus includes a bioreactor for containing a mixture of wastewater under treatment and activated sludge. The bioreactor is divided into a plurality of serially connected treatment zones and includes a wastewater inlet, a downstream aerobic zone and an upstream aerobic zone between the wastewater inlet and the downstream aerobic zone. A membrane filter is provided in the downstream aerobic zone so that it functions as an immersed membrane filter with a bioreactor containing an operative volume of wastewater and activated sludge. The immersed membrane filter filters treated water flowing from the bioreactor through a first outlet. An aerator is operatively associated with the membrane filter for purging solids from the membrane filter with air or a gas including a select concentration of oxygen. A second outlet in the downstream aerobic zone is connected to an inlet in the upstream aerobic zone for recycling activated sludge charged with oxygen from the downstream aerobic zone to the upstream aerobic zone.
A second aspect of the invention includes a treatment tank having an inlet and a downstream outlet with the inlet coupled to the first outlet of the bioreactor for receiving treated water. A powdered activated carbon (xe2x80x9cPACxe2x80x9d) supply provides PAC to the treatment tank near the treatment tank inlet. A membrane filter is operatively associated with the downstream outlet and is situated so that it functions as an immersed membrane filter with the treatment tank containing an operative volume of treated water. A coagulant supply may also be operatively associated with the treatment tank for providing coagulant to the treatment tank near the treatment inlet. The treatment tank preferably includes a plurality of mixing baffles between the inlet and the outlet. A monitor may be provided for monitoring a concentration of dissolved organics in the treated water flowing into the receiving tank inlet. The monitor is operatively associated with the PAC supply to vary the amount of PAC provided to the treatment tank as a function of the concentration of dissolved organics in the treated water.
A third aspect of the present invention is an apparatus utilizing activated sludge for the removal of biological nutrients from a wastewater including a bioreactor having a wastewater inlet and a downstream treated water outlet for containing a mixture of wastewater under treatment and activated sludge and for flowing the mixture along a treatment path between the bioreactor inlet and outlet. A membrane filter is operatively associated with the bioreactor outlet and is situated so that it functions as an immersed membrane filter with the bioreactor containing an operative volume of wastewater and activated sludge. A treatment tank has an inlet coupled to the bioreactor for receiving treated water and a downstream outlet. A PAC supply is operatively associated with the treatment tank for providing PAC to the treatment tank near the treatment tank inlet. A membrane filter is operatively associated with the downstream outlet and is situated so that it functions as an immersed membrane filter with the treatment tank containing an operative volume of treated water.
A fourth aspect of the present invention is a method for the removal of nutrients from a wastewater including providing a wastewater to an inlet of a serial, multi-zone, activated sludge bioreactor containing activated sludge. The bioreactor has a downstream aerobic zone from which treated water is removed and an upstream aerobic zone between the wastewater inlet and the downstream aerobic zone. The method further includes filtering treated water from the activated sludge in the downstream aerobic zone through an immersed membrane filter. Solids are purged from the immersed membrane filter with an oxygen containing gas and return activated sludge charged with oxygen is recycled from the downstream to the upstream aerobic zone. The method may further include feeding treated water from the immersed membrane filter to a treatment tank inlet, adding PAC to the treatment tank near the treatment inlet, flowing the treated water and PAC along a treatment path within the treatment tank and filtering the treated water from the PAC through an immersed membrane filter. The method may also include adding a coagulant or an oxidant to the treatment tank near the treatment tank inlet. A population of microorganisms for metabolizing dissolved organic material may also be maintained in the treatment tank. In a preferred embodiment, the PAC is added on a continuous basis and the concentration of dissolved organic material in the treated water fed into the treatment tank is monitored so that the amount of PAC added to the treatment tank is varied as a function of the concentration of dissolved organic material in the treated water.
The first, second and fourth aspects of the present invention provide for recycle of oxygen charged return activated sludge from an aerobic zone containing the immersed membrane filter to an upstream aerobic zone. This recycle not only re-disburses microorganisms which metabolize dissolved organic materials, but also provides a source of oxygenated return activated sludge to the upstream aerobic zone which decreases the need to provide supplement oxygen to the upstream aerobic zone. This allows the method and apparatus for wastewater treatment to function more efficiently. The use of the immersed membrane filter allows for a higher concentration of MLSS within the bioreactor which allows the volume requirements for the activated sludge process to be reduced. The use of the immersed membrane filter also provides a physical barrier to pathogens such as Giardia and Cryptosporidium which is absent in conventional treatment processes using gravity separators. Those embodiments having a treatment tank charged with PAC receiving the effluent from the bioreactor and including an immersed membrane filter on the tank outlet provide a xe2x80x9cmultiple barrierxe2x80x9d to assure contaminants such as suspended solids, turbidity and pathogens will be adequately removed from the plant effluent. Furthermore, the addition of PAC to the downstream treatment tank provides for greater removal of dissolved organics. The use of PAC in the treatment tank allows for addition of increased concentrations of PAC in the event of a spike of organics introduced into the treatment process, as may result from a toxic spill event. Treatment processes using conventional granular-activated carbon filters do not provide this flexibility. In addition, the porous structure of the PAC provides many sites for microbial growth. Promotion of microorganisms within the treatment tank will provide for oxidation of organics within the treatment tank in addition to absorption by the PAC. These many advantages result from a novel combination of proven treatment apparatus and can be provided economically and dependably.